1. Field of the Invention
The present invention relates to a method and apparatus for equalizing the frequency response (frequency characteristic) of an FM video signal (frequency modulated video signal) using a variable equalizer and a video recording and reproducing apparatus.
2. Description of the Related Art
With video recording and reproducing equipment represented by video tape recorders (VTRs), a video signal is generally recorded on magnetic tape by a magnetic head in the form of an FM signal (frequency modulated signal). The FM video signal is obtained by frequency-modulating a carrier signal whose frequency is above the baseband of the video signal with a video signal. The FM video signal reproduced from the magnetic tape by a reproduce head is amplified by a preamplifier to an adequate level for the subsequent signal processing and then entered into an equalizer, which compensates for variations in the frequency response of the FM video signal which result from the processes of recording and reproducing the FM signal. The operation of compensation is called the equalization. The FM video signal equalized by the equalizer is applied to an FM demodulator where the baseband video signal is recovered.
The role of the equalizer will be explained in more detail. When passing through the record/reproduce system, the FM video signal is influenced by the gap loss associated with the gap length of the magnetic head and the space loss associated with the distance between the magnetic head and the magnetic tape. The reproduced FM video signal thus has such a low-frequency emphasized response as shown in FIG. 1 in which it is nonlinear within the FM band (frequency band for recording) and the amount of attenuation increases as frequency increases. When the reproduced FM video signal having such a frequency response is directly applied to the FM demodulator for demodulation, the resultant baseband video signal will have no flat frequency response and generally has its high frequency components raised. The reproduced FM video signal is equalized by an equalizer in order to flatten the frequency response of the baseband video signal to be demodulated.
To flatten the frequency response of the demodulated baseband video signal, it is required to adjust the frequency response of the equalizer properly. In general, the frequency response of the equalizer is adjusted before the shipment of VTR and then fixed. With some VTRs of studio type, a user can adjust the frequency response of the equalizer with a trimmer.
In VTRs, the frequency response of a reproduced FM video signal varies because of variations in a recording current, the wear of magnetic heads, a difference in azimuth angle between recording and reproducing heads and variations in head-to-tape spacing. When, therefore, the frequency response of the equalizers is fixed, the frequency characteristics of the equalized FM video signals are not always flat but vary. Such variations in the frequency characteristics are allowed to some extent in household VTRs, but must be suppressed to a minimum in high-definition VTRs which are now being developed. With existing VTRs of studio type, special test signals, such as multi-burst signals or frequency-sweep signals, are used to monitor the frequency response of the reproduced baseband video signal so that the user may adjust the response of the equalizer using the trimmer. However, it is very complicated and time-consuming to perform such manual adjustment each time the VTR is put to use.
Techniques are known which monitor the frequency response of a reproduced FM signal using low-frequency and high-frequency signal components in a reproduced signal to automatically adjust the response of a variable equalizer. For example, in published unexamined Japanese patent applications Nos. 62-248107 and 62-248108, the downshifted frequency converted chroma signal is used as the low-frequency signal component and the frequency-modulated horizontal-sync signal is used as the high-frequency signal component in VTRs using the color-under recording system. In playback, both the signals are compared with each other in magnitude to detect the frequency response of the reproduced FM video signal and to control the variable equalizer accordingly. The recording levels of the downshifted frequency converted chroma signal and the frequency-modulated horizontal sync signal are not necessarily constant. It is thus difficult to correctly monitor the frequency response of the reproduced FM video signal by comparison in level between the signals. Hence, the equalizer cannot necessarily be controlled properly. Also, the techniques cannot be applied to studio-type VTRs and high-definition VTRs using systems other than the color-under recording system.